As vehicles are propelled into earth orbit, their propulsion systems require engines which are driven with self contained oxidizers, i. e. rocket engines. Rocket engines are required for a number of reasons.
First of all, a vehicle entering earth orbit must be accelerated even after the vehicle leaves the atmosphere of the earth. Thus an engine driving such a vehicle must have a self-contained source of oxygen to facilitate combustion of its fuel.
Secondly, there is a need to provide a very high thrust to weight ratio for the vehicle in order to facilitate vertical liftoff and rapid acceleration. Rocket engines are well suited to this task.
However, a rocket propelled vehicle must have a large portion of its initial or take-off mass dedicated to fuel and on-board oxidizer. In other words, rocket engines have very low specific impulse, i.e., a low ratio of thrust to mass of on board oxidizer and fuel. A consequence of this inherently low specific impulse is that earth to orbit vehicles must be propelled into orbit using multiple stage rockets. Heretofore, there has been no practical way to propel a vehicle into earth orbit with a single stage launch vehicle. This need to use multiple stage rockets has, of course, precluded development of a reusable launch vehicle for earth orbit devices.
Development efforts have been directed to achieving a practical reusable launch vehicle for earth orbit devices. A number of prior art proposals have been made to utilize an air breathing engine to produce some of the needed thrust for an earth to orbit vehicle while the vehicle is still in the earth's atmosphere.
These prior art efforts have encountered one common difficulty. Turbojet engines are inherently heavy when compared to rocket engines. In other words, turbojet engines have a much lower thrust to weight ratio than rocket engines. As a result of this inherent feature of conventional turbojet engines, it is difficult to enhance payload performance of a launch vehicle by adding a turbojet engine to the vehicle. In many cases, the added weight of the turbojet engine exceeds any payload advantage which would result from reduction of weight derived from reducing on-board oxidizer.
There have been some prior art efforts directed to increasing the thrust to weight ratio of turbojet engines. Engines which operate with cryogenic fuels such as liquid hydrogen have been developed. These cryogenic engines utilize incoming-air pre-cooling as a mechanism to avoid overheating of engine components. If engine components can be operated at reduced temperature, they can be fabricated from relatively light alloys. Thus, a turbojet engine with effective pre-cooling can become an engine with an improved thrust to weight ratio.
In prior art turbojet engines, this pre-cooling is produced by a heat exchanger which employs the cooling effect of liquid hydrogen fuel as it passes from its on-board storage to a combustion chamber on the vehicle. While this system is somewhat effective in improving thrust-to-weight ratio, it does not improve this ratio sufficiently to add significantly to the payload performance of a launch vehicle. Some examples of these proposals are found in U.S. Pat. No. 5,101,622 (Bond) and an article entitled "Air Precooling for Aerospace Engine: Soviet Style", A. Rudakov and V. Balepin, Aerospace Engineering, Aug. 1991, pp.29-30, (Ref. 2).
Other efforts have been made in the prior art to improve the thrust to weight ratio. Additional cooling is provided by overfueling the engine. The pre-cooled turbojet engine is driven with more hydrogen than is needed for stoichiometric operation. This passes more hydrogen through the pre-cooling system which produces greater cooling. This provides for somewhat higher thrust capability with light weight alloy engine components. But, this technique is subject to the rule of diminishing returns. Increased thrust is derived from use of excessive hydrogen. Excessive hydrogen use has its payload costs. More hydrogen must be put on board the vehicle and its storage tanks must be larger. Consequently, there is very little net improvement in thrust to weight ratio when the excess hydrogen cooling technique is employed.
Most of the prior art development efforts aimed at achieving a practical re-usable earth to orbit launch vehicle have focused on the use of air-breathing engines to augment rocket engines. These developments efforts have heretofore been frustrated because of the difficulties presented by the inherently low thrust to weight ratios of turbojet engines.
It is a goal of the present invention therefore to provide a practical propulsion system for re-usable earth to orbit vehicle which utilizes an air breathing engine combined with a rocket engine.
It is a further goal of the present invention to provide such a propulsion system by employing a unique set of operating techniques for combined turbojet and rocket engines.